LibWeb: Fire "load" and "error" events for rel=preload link loads

Author: https://github.com/awesomekling
Commit: b97004ea4a
Pull-request: https://github.com/SerenityOS/serenity/pull/19275
Reviewed-by: https://github.com/gmta ✅

Tests/LibWeb/Text/expected/link-element-rel-preload-load-event.txt

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+Got load event
+Got error event

Tests/LibWeb/Text/input/link-element-rel-preload-load-event.html

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+<script src="include.js"></script>
+<script>
+    let goodLink = document.createElement("link");
+    goodLink.setAttribute("rel", "preload");
+    goodLink.setAttribute("href", "valid.css");
+    goodLink.setAttribute("as", "style");
+    goodLink.addEventListener("load", function() {
+        println("Got load event");
+    });
+    document.head.appendChild(goodLink);
+
+    let badLink = document.createElement("link");
+    badLink.setAttribute("rel", "preload");
+    badLink.setAttribute("href", "this-file-does-not-exist-and-so-is-invalid.css");
+    badLink.setAttribute("as", "style");
+    badLink.addEventListener("error", function() {
+        println("Got error event");
+    });
+    document.head.appendChild(badLink);
+</script>

Tests/LibWeb/Text/input/valid.css

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+* { }